1. Field of the Invention
The present invention relates to a process for producing polyethylene having impact resistance and to a catalyst for use in such a process.
2. Description of the Prior Art
Polyethylene is known for use in the manufacture of a wide variety of articles. The polyethylene polymerization process can be varied in a number of respects to produce a wide variety of resultant polyethylene resins having different physical properties which render the various resins suitable for use in different applications. In particular, it is known to use polyethylene for use in applications where the polyethylene is required to have impact resistance. For example, polyethylene is known for use in the manufacture of pipes where it is obviously required that the material of the pipe has sufficient impact resistance so as to avoid inadvertent fracture in service. Furthermore, polyethylene is known for use as a film where the property of impact resistance of the film can be of importance.
Chromium-based catalysts used for the production of polyethylene have been known for some time. The catalysts may have been subjected to pre-treatment processes prior to the polymerization reaction. The pre-treatment processes may include chemical reduction of the chromium to a lower valence state, e.g. by carbon monoxide. Such a process is disclosed in, for example, EP-A-0591968.
U.S. Pat. No. 5,208,309 discloses a linear, very low density polyethylene polymerization process. In the polymerization process, a cocatalyst such as a boron alkyl, most particularly triethyl borane (TEB), is employed which tends to generate an in situ comonomer and thus depresses the density of the resultant polyethylene resin. The catalyst support is a silica-titania support and for producing a polymer with the most desirable characteristics it is preferred for the support to contain 5 to 8 wt % titanium. The specification does not address the problem of the production of polyethylene having impact resistance.
U.S. Pat. No. 4,151,122 discloses the reduction and reoxidation of a cogel or self-reduced catalyst in which oxidation of a catalyst is employed after reduction of a catalyst, such as by carbon monoxide, for boosting the melt index of the resultant polyethylene resin. There is no disclosure of a process for producing polyethylene having impact resistance.
U.S. Pat. No. 4,405,768 discloses the addition of titanium to a catalyst for boosting the melt index of the polyethylene resin. Again, there is no disclosure of a process for producing polyethylene having impact resistance.
EP-A-0000751 and its equivalent U.S. Pat. No. 4,180,481 disclose a process for the manufacture of a supported chromium oxide catalyst for olefin polymerization, the catalyst having been treated with carbon monoxide. There is no disclosure of a process for producing polyethylene having impact resistance.